Blade pitch control in a helicopter main rotor typically is achieved through the use of a swashplate assembly for transmission of control inputs from the stationary control system to the rotating system. Typically a spherical mount (or uniball mount) centers one of the swashplate ring members and accommodates tilt and axial translation of the swashplate assembly. A scissors linkage is provided for driving or rotating a swashplate member with the main rotor shaft and for maintaining the proper alignment of the control rods which impart pitch motion to the rotor blades.
The rotating scissor linkage is comprised of two metal links pinned or hinged together at one end, and forming a V-shape. The opposing ends of the links are pin mounted to the rotor shaft at one end and articulately mounted by a ball and socket fitting to the rotating swashplate ring at the other end. The swashplate assembly has proven to be a very reliable and relatively maintenance-free configuration. In contrast, the rotating scissors linkage accounts for a relatively high percentage of the necessary repairs and maintenance associated with that portion of the control system involving the swashplate assembly and the rotor hub. The bearings of the scissors linkage, particularly the spherical bearing connection to the rotating swashplate ring, wears much more rapidly than other bearings in the swashplate assembly due to the large once per revolution angular motion the pivot bearings experience when the swashplate is tilted. Also, vibratory loads are induced due to small azimuthal motions of the swashplate induced by the geometric coupling of scissor and swashplate motions. Finally, Coriolis loads are applied with each revolution. This is due to the radial changes in the center of gravity of the scissors linkage as the linkage opens and closes, thus causing a change in angular momentum. Accordingly, the rotating swashplate ring accelerates and decelerates once per revolution thereby accelerating the rate of wear to the scissors linkage bearings. It is desirable, therefore, to provide a swashplate assembly which eliminates the need for a scissors linkage.
Patents directed to helicopter rotor drives and which do not include a scissors linkage include U.S. Pat. No. 3,558,082 and U.S. Pat. No. 4,588,355, the latter patent being assigned to Applicants' assignee.